Ask the Vet: Broodmare Practices

Courtesy of AAEP
Answered by, Holly Mason, DVM, MS, Utah State Veterinary 

 I have a 2-year-old Molly mule that shows strong estrus behaviors. Will implanting marbles in her uterus help? Or are herbal supplements or other therapies preferred? Cost is a factor.

Answer: I have a real soft spot for these creatures and I applaud you for being a mule owner. Your situation is not unique to molly mules. Dealing with undesirable estrus behavior can be a problematic situation for the owner of any female equid.

Marbles have been used to suppress estrus with mixed results. Some authors report that timing the placement of the marble as close to ovulation as possible has a positive influence on the efficacy. The uterus is also surprisingly good at expelling the marble. These reasons have made marble use overwhelmingly unpopular among horse owners and veterinarians. That being said, the marble would likely be your least cost interventional option.

The most reliable method to suppress estrus is by administering synthetic progesterone. You have two options in this category. The first and most reliable is to administer an oral solution (Regumate) daily. This can be costly and there are human health risks associated with exposure to this product. The second is by administering an intramuscular injection of a compounded long acting progesterone. Depending on the formulation, the injection may need to be repeated every 2-4 weeks. My experience has been that the injectable method is slightly less effective than the oral method. A lot of what determines the treatment depends on the client’s budget and level of expectation. I have used both methods with very acceptable results.

You may hear some people talk about spaying a molly mule. This is a procedure during which the ovaries are removed. I would caution you against this procedure as it rarely eliminates the estrus behavior and in fact often makes it worse.

A no cost method of dealing with your molly mule’s estrus would be to track her cycles on the calendar and limit or reduce your demands on her during times when you know she will be in heavy heat. As a point of reference, most mares are seasonal breeders that begin cycling in early spring and go into winter anestrus around late fall. Ovulation occurs about every 21 days and heat is evident for 5-7 days around the time of ovulation.